1. Field of the Invention
The present invention relates to pneumatic percussion devices, and more particularly to pneumatic burrowing devices of the type which have a projectile-like outline and which propel themselves through the ground by displacing the materials which are in their path.
2. Description of the Prior Art
Several types of self-propelled pneumatic burrowing devices -- sometimes also called ground torpedos or ground rockets -- are known from the prior art. They all have in common that a heavy percussion plunger is arranged for longitudinal movement inside a cylindrical casing which carries a percussion head with a burrowing tip on its forward extremity and a supply line for compressed air on its rear extremity. The reciprocating movements of the percussion plunger, created by the action of the compressed air, cause the plunger to strike the percussion head at the end of its forward stroke, the rearward movement of the percussion plunger being obtained by temporarily opening air flow channels which lead the compressed air to the forward side of the percussion plunger and which are closed again, before the plunger reaches its rearward point of motion reversal. The hard impact of the percussion plunger against the percussion head drives the latter forwardly against the materials in its path be they soft ground or rock, the distance of penetration per impact varying from several millimeters to a fraction of a millimeter, depending upon the resistance encountered. The percussion head has a shoulder portion of a diameter which corresponds to the diameter of the casing of the device, so that the latter will follow the advance of the percussion head in reaction to the reversal of the percussion plunger motion, following its impact against the percussion head.
Burrowing devices of this type are thus capable of advancing through solid ground or rock, producing a substantially straight-line cylindrical channel of a diameter equal to the diameter of the casing of the device. This burrowing method is particularly suitable for the placement of underground cables and protective tubing for conduits such as water and/or gas supply lines, as well as sewerage lines, which can thus be installed without the need for digging any installation ditches. The method is particularly suitable for situations where channels have to be produced underneath buildings and structures, such as a street pavement, retaining walls, and other existing above-ground structures.
The burrowing devices in question are thus self-propelled and self-guided, and once pentrated into the ground are inaccessible for servicing or adjustment until they emerge at the point of destination, or are retrieved at the point of penetration. The devices, therefore, need to be very robust and operationally reliable, in addition to having a high degree of guidance accuracy, in order to emerge at the desired point of destination. Also, there may exist situations under which a burrowing device becomes stuck underground, in which case it becomes necessary to restart or reverse the movement of the device. Without this capability, the device could be irretrievably lost underground. Such self-propelled burrowing devices with restart and reversing features are known from the prior art. One such device is described in U.S. Pat. No. 3,865,200.
Another important operational problem complex of self-propelled burrowing devices relates to the configuration of the percussion head and to the interaction between this head and the percussion plunger. In one known prior art device, the percussion head has a pointed penetration tip in the shape of a projectile tip, the head being solidary with the cylindrical casing, either rigidly attached thereto, or formed as an integral portion of the latter. This rigid connection accounts for the fact that the casing of this type of burrowing device is subjected to elevated impact stress conditions which tend to produce fissures in the casing wall. Accordingly, the longevity of this type of device is a comparatively short one.
In order to avoid the aforementioned stress conditions on the casing of the burrowing device, it has therefore already been suggested that the percussion head should have a limited axial mobility in relation to the casing, while a spring is arranged between the casing and the percussion head so as to urge the latter rearwardly against the casing. The result is that the impact of the percussion plunger against the rearwardly biased percussion head is not immediately transmitted to the casing, but causes the percussion head to advance by compressing the spring which then assists the compressed air in pushing the casing forwardly so as to follow the advance of the percussion head. This arrangement greatly reduces the stress which is created on the casing of the device, while increasing its operational longevity correspondingly. The provision of an adjustable preload on the spring between the percussion head and the casing further makes it possible to adapt the kinematic interaction between the percussion head and the casing to different operational conditions in accordance with the burrowing resistance encountered in the ground.
It has been found, however, that the arrangement of the percussion head as a longitudinally displaceable, spring-biased member of the burrowing device has certain operational shortcomings which manifest themselves primarily in the form of a reduced accuracy of straight-line guidance and in a less favorable utilization of the percussion energy. The lower energy efficiency is due to the fact that a portion of the initial impact energy which is transferred from the percussion plunger to the percussion head is absorbed by the spring, to be released later, when it either advances the casing or retracts the percussion head.
The spring displacement, in order to produce an appreciable damping action, must be at least 10 mm, meaning that the axial displaceability of the percussion head relative to the casing must be equal to that distance. The actual penetration advance, however, varies between 0.1 and 3 mm per plunger impact, so that there exists the possibility that, due to friction between the ground and the casing, the latter will not advance at all, or only by a lesser distance than the percussion head, and that the percussion head will subsequently be retracted from its advanced position through the action of the spring. If this happens, then the initial impact of the percussion plunger against the percussion head causes the latter to rebound forwardly until it reaches its previous most advanced position, thereby dissipating a portion of the impact energy.
The aforementioned tendency of this prior art device to retract the percussion head from its advance position leads to a hammering action between the percussion head and the casing which may lead to an axial "dancing" movement of the entire burrowing device, especially when the surrounding ground is soft and the percussion head suddenly encounters a hard layer of material such as rock or a block of concrete, for example. This dancing movement of the burrowing device in soft ground produces an enlarged, often elliptical, channel cross section around the casing of the device, with the result that its straight-line guidance is no longer accurately maintained and that the device may accordingly deviate from its intended direction of advance.